There are a number of interpretations of what one means about probability. For example, a frequency account of probability might mean that in all possible worlds, God is likely to exist in most of the possible worlds. However, this gives us no clue if we happen to exist in one of the universe's that God does exist in. Another example might be a rational account of probability, God is to be considered to most likely exist in our universe if you consider the evidence in the most rational manner. The problem with this account is that you need to believe in the 'rational manner' to see God's existence as likely (translation: you have to be rational to be rational).

There are other ways to construe probability statements, but they all have problems when talking about ontological commitments. I guess Dick would say that I am being cavil.